The present invention relates to a thermal overload relay for use in combination with an electromagnetic contactor, and in particular, to an inversion structure for stabilizing an inversion operation performed by the thermal overload relay.
To protect an electric motor from an overload, there is employed, as a standard distribution method, a method of combining a thermal overload relay with an electromagnetic contactor connected to a power circuit for the electric motor and allowing the electromagnetic contactor to cut of f current in an overload operation to stop the electric motor.
FIGS. 4 and 5 show a conventional thermal overload relay. FIG. 4 is a view of an internal mechanism of the thermal overload relay showing a steady or general state, and FIG. 5 is a view of the internal mechanism of the thermal overload relay showing an overload state.
In these figures, 1 is a main body case, 2 is a main bimetal (only one phase of a three-phase circuit is shown), 3 is a shifter coupled to a tip of the main bimetal 2, 4 is an inversion operation mechanism for opening or closing a contact, 5 is a normally closed contact comprising a movable contact 5a and a fixed contact 5b, 6 is a normally open contact comprising a fixed contact 6a and a movable contact 6b, 7 is a releasing lever journaled for rotational movement around a support point 7b to link the shifter 3 of the main bimetal 2 and the inversion operation mechanism 4 together, 8 is an adjustment link having a lower end supported in a slot 1a formed in the main body case 1, the adjustment link being coupled to a proximal end of the releasing lever 7 at the support point 7b, and 9 is an adjustment dial having a cam surface 9a on which an upper end 8a of the adjustment link 8 contacts.
Further, the inversion operation mechanism 4 comprises a pivotally movable plate 10 having one end locked and supported in a V-shaped groove 11a of a generally U-shaped support piece 11, a tension spring 12 (inversion driving spring) extending between a tip portion 1a of the movable plate 10 and a spring catching section 11b of the support piece 11, and a normally-open-contact driving lever 13 projecting backward from the movable plate 10 in the form of the character L. The movable plate 10 has the movable contact 5a of the normally closed contact 5 attached to a tip portion thereof. Additionally, the fixed contact 5b of the normally closed contact 5 is attached to a contact supporting piece 14 with a flat spring structure, having one end fixed to a bottom portion of the main body case 1 so as to lie horizontally with respect to the relay.
The tension spring 12 has a coil-like spring section 12b formed of a wire of a spring steel material and has hook sections formed at opposite ends thereof. The releasing lever 7 has a lever tip portion 7a in a circular form abutting against the middle of the wire of the tension spring 12.
With such a configuration, in a steady or normal state as shown in FIG. 4, the movable plate 10 of the inversion operation mechanism 4 is tilted clockwise from its neutral position under a force from the tension spring 12, and the movable contact 5a of the normally closed contact 5 connected in series with an electromagnetic coil of an electromagnetic contactor is pressed against the fixed contact 5b to maintain the contacts in the xe2x80x9conxe2x80x9d state. In this state, the normally closed contact 6 is off.
Then, when overcurrent flows through a main circuit, the main bimetal 2 is heated and bent, and thus has its free end displaced to move the shifter 3 rightward. The releasing lever thus pivots around the support point 7b from a position shown by the dotted line to a position shown by the solid line as shown in FIG. 5. At this point, the middle of the wire of the tension spring 12 of the inversion operation mechanism 4 is pushed upward by the lever tip portion 7a. When the displacement of the tension spring exceeds a dead point of the movable plate 10, the movable plate 10 is rapidly driven to be inverted to separate the movable contact 5a of the normally closed contact 5 from the fixed contact 5b, and the drive lever 13 presses a movable contact shoe piece with the movable contact 6b attached thereto to bring the movable contact 6b to contact with the fixed contact 6a to turn on the contact.
Next, a method for adjusting a setting current value of the overcurrent relay will be described with reference to FIG. 4. In this figure, when the adjustment dial 9 is rotated, the adjustment link 8 with the upper end 8a, which abuts against the cam surface 9a, is displaced around the slot 1A in the case from a position shown by the solid line to a position shown by the dotted line. In connection with this, the releasing lever 7 coupled to the adjustment link 8 can be displaced and moved from a position shown by the solid line to a position shown by the dotted line to change the gap between the releasing lever 7 and a tip of the shifter 3. Further, this operation for adjusting the setting current value causes the lever tip portion 7a of the releasing lever 7 to move in a direction A (rightward) in the figure, wherein a movement range of the lever tip portion 7a is denoted by C.
The thermal overload relay of the above-described conventional structure has problems in operational characteristics as described below.
When the setting current value is adjusted by rotating the adjustment dial 9 as described above, the position of the lever tip portion 7a of the releasing lever 7 abutting against the wire of the tension spring 12 moves in the direction A along the middle of the wire of the tension spring 12 in the movement range C. When the lever tip portion 7a shifts in a lateral direction along the spring wire, the apparent lateral rigidity of the tension spring 12 changes, thereby changing an inversion operation characteristic of the movable plate 10. That is, when the position of the lever tip portion 7a moves in the direction A relative to the wire, the distance between the lever tip portion and the upper end 1a of the tension spring 12 decreases to increase the apparent lateral rigidity of the spring, to thereby reduce the flexion of the tension spring 12b when it is pushed by the shifter 3 via the releasing lever 7, the shifter 3 following the bending of the main bimetal 2. Consequently, the movable plate 10 of the inversion operation mechanism 4 can not be rapidly inverted but is slowly displaced upward.
It is an object of the present invention to solve the above problems and to provide an improved thermal overcurrent relay that achieves a stable inversion operation and stable characteristics thereof regardless of an adjustment of the setting current value.
To attain the above object, the present invention provides a thermal overload relay including an inversion operation mechanism that is driven by a releasing lever to open or close a contact. The inversion operation mechanism comprises a movable plate supported so as to be inverted by using one end thereof as a support point, and a tension spring for driving the movable plate for inversion. The releasing lever presses the middle of a wire of the tension spring to drive the movable plate for inversion. In the invention, a projection is formed in the middle of the wire of the tension spring to abut against the releasing lever.
According to the invention, even when the adjustment dial is used to displace the releasing lever via the adjustment link, since the position of the projection formed in the middle of the wire of the tension spring remains unchanged, a stable inversion operation is obtained.
Further, in the above-described thermal overload relay, if the projection formed on the tension spring deviates from an axis joining opposite ends of the tension spring together, the deflection of the tension spring is suppressed to enable a more stable inversion operation.